1. Field of the Invention
This invention relates to an over-coating agent for forming fine patterns in the field of photolithographic technology and a method of forming fine-line patterns using such agent. More particularly, the invention relates to an over-coating agent for forming or defining fine-line patterns, such as hole patterns and trench patterns, that can meet today's requirements for higher packing densities and smaller sizes of semiconductor devices.
2. Description of the Related Art
In the manufacture of electronic components such as semiconductor devices and liquid-crystal devices, there is employed the photolithographic technology which, in order to perform a treatment such as etching on the substrate, first forms a film (photoresist layer) over the substrate using a so-called radiation-sensitive photoresist which is sensitive to activating radiations, then performs exposure of the film by selective illumination with an activating radiation, performs development to dissolve away the photoresist layer selectively to form an image pattern (photoresist pattern), and forms a variety of patterns including contact providing patterns such as a hole pattern and a trench pattern using the photoresist pattern as a protective layer (mask pattern).
With the recent increase in the need for higher packing densities and smaller sizes of semiconductor devices, increasing efforts are being made to form sufficiently fine-line patterns and submicron-electronic fabrication capable of forming patterns with linewidths of no more than 0.20 μm is currently required. As for the activating light rays necessary in the formation of mask patterns, short-wavelength radiations such as KrF, ArF and F2 excimer laser beams and electron beams are employed. Further, active R&D efforts are being made to find photoresist materials as mask pattern formers that have physical properties adapted to those short-wavelength radiations.
In addition to those approaches for realizing submicron-electronic fabrication which are based on photoresist materials, active R&D efforts are also being made on the basis of pattern forming method with a view to finding a technology that can provide higher resolutions than those possessed by photoresist materials.
For example, JP-5-166717A discloses a method of forming fine patterns which comprises the steps of defining patterns (=photoresist-uncovered patterns) in a pattern-forming resist on a substrate, then coating over entirely the substrate with a mixing generating resist that is to be mixed with said pattern-forming resist, baking the assembly to form a mixing layer on both sidewalls and the top of the pattern-forming resist, and removing the non-mixing portions of said mixing generating resist such that the feature size of the photoresist-uncovered pattern is reduced by an amount comparable to the dimension of said mixing layer. JP-5-241348 discloses a pattern forming method comprising the steps of depositing a resin, which becomes insoluble in the presence of an acid, on a substrate having formed thereon a resist pattern containing an acid generator, heat treating the assembly so that the acid is diffused from the resist pattern into said resin in-soluble in the presence of an acid to form a given thickness of insolubilized portion of the resist near the interface between the resin and the resist pattern, and developing the resist to remove the resin portion through which no acid has been diffused, thereby ensuring that the feature size of the pattern is reduced by an amount comparable to the dimension of said given thickness.
However, in these methods; it is difficult to control the thickness of layers to be formed on the sidewalls of resist patterns. In addition, the in-plane heat dependency of wafers is as great as ten-odd nanometers per degree Celsius, so it is extremely difficult to keep the in-plane uniformity of wafers by means of the heater employed in current fabrication of semiconductor devices and this leads to the problem of occurrence of significant variations in pattern dimensions.
Another approach known to be capable of reducing pattern dimensions is by fluidizing resist patterns through heat treatment and the like. For example, JP-1-307228A discloses a method comprising the steps of forming a resist pattern on a substrate and applying heat treatment to deform the cross-sectional shape of the resist pattern, thereby defining a fine pattern. In addition, JP-4-364021A discloses a method comprising the steps of forming a resist pattern and heating it to fluidize the resist pattern, thereby changing the dimensions of its resist pattern to form or define a fine-line pattern.
In these methods, the wafer's in-plane heat dependency is only a few nanometers per degree Celsius and is not very problematic. On the other hand, it is difficult to control the resist deformation and fluidizing on account of heat treatment, so it is not easy to provide a uniform resist pattern in a wafer's plane.
An evolved version of those methods is disclosed in JP-7-45510A and it comprises the steps of forming a resist pattern on a substrate, forming a stopper resin on the substrate to prevent excessive thermal fluidizing of the resist pattern, then applying heat treatment to fluidize the resist so as to change the dimensions of its pattern, and thereafter removing the stopper resin to form or define a fine-line pattern. As the stopper, resin, a water-soluble resin, specifically, polyvinyl alcohol is employed singly. However, polyvinyl alcohol alone is not highly soluble in water and cannot be readily removed completely by washing with water, introducing difficulty in forming a pattern of good profile. The pattern formed is not completely satisfactory in terms of stability over time. In addition, polyvinyl alcohol cannot be applied efficiently by coating. Because of these and other problems, the method disclosed in JP-7-45510 has yet to be adopted commercially.
For solving these prior-art problems, the present applicant has proposed a technique directed to an over-coating agent for forming fine patterns and to a method of forming fine patterns in JP 2003-084459A, JP 2003-084460A, JP 2003-107752A, JP 2003-142381A, JP 2003-195527A, and JP 2003-202679A, etc. The technique shown in these patent publications has made it possible to form fine-line patterns that satisfy pattern dimension controllability, good profile and other necessary properties for semiconductor devices.
In the technique of forming fine-line patterns using the above over-coating agent for forming fine patterns, a photoresist layer is first formed on a substrate and this is exposed to light and developed to form a photoresist pattern (mask pattern). Next, the over-coating agent for forming fine patterns is applied to cover the entire surface of the substrate, and then this is heated, whereby the width of the photoresist pattern lines is enlarged by utilizing the thermal shrinking effect of the over-coating agent for forming fine patterns, and, as a result, the distance between the adjacent photoresist pattern lines is thereby narrowed and the width of the pattern line (of various patterns such as hole pattern, and trench pattern) to be determined by the distance between the photoresist pattern lines is also narrowed to give finer trace patterns.
The above-mentioned forming fine-line pattern process undergoes the influence of pattern dimension control in two stages: that is, a photoresist patterning stage (first stage) and a thermal shrinking stage of the over-coating agent for forming fine patterns (second stage). In the process comprising said two stages, when photoresist patterning is performed by increasing the luminous exposure of light to which a photoresist is exposed in the first stage, the degree of thermal shrinkage of the over-coating agent in the second stage is apt to be larger than the expected degree thereof and, as a result, it is often difficult to anticipate the dimension controllability in forming fine-line patterns in the process.
In that situation, it is desirable that the degree of thermal shrinkage of the over-coating agent could be kept constant even when the luminous exposure is varied relative to CD (critical dimension) of the photoresist pattern just after development.
In addition, it is also desirable that, even when various patterns differing in the pattern dimension and the line-to-line distance exist on one substrate, all the patterns could enjoy the same degree of thermal shrinkage.
In view of the shape of photoresist patterns, even when photoresist patterns having a good rectangular cross-sectional profile could be formed in the photoresist patterning stage (first stage), there may occur in the second stage a problematic phenomenon that the top of the photoresist pattern may be rounded owing to the thermal shrinkage of the over-coating agent applied thereto for forming fine patterns. In particular, when an ArF photoresist is used for forming finer line patterns, the top of the photoresist pattern could not keep the original rectangular profile thereof but is often rounded in the thermal shrinkage step for the over coating agent applied onto the photoresist pattern.
Accordingly, an over-coating agent for forming fine patterns has been studied and developed, which ensures a high degree of thermal shrinkage and a large exposure margin and enables pattern dimension control while keeping a good photoresist pattern profile as such.
JP 2001-281886A discloses a method comprising the steps of covering a surface of a resist pattern with an acidic film made of a resist pattern, size reducing material containing a water-soluble resin, rendering the surface layer of the resist pattern alkali-soluble, then removing said surface layer and the acidic film with an alkaline solution to reduce the feature size of the resist pattern. JP-2002-184673A discloses a method comprising the steps of forming a resist pattern on a substrate, then forming a film containing a water-soluble film forming component on said resist pattern, heat treating said resist pattern and film, and immersing the assembly in an aqueous solution of tetramethylammonium hydroxide, thereby forming a fine-line resist pattern without involving a dry etching step. However, both methods are simply directed to reducing the size of resist trace patterns themselves and therefore are totally different from the present invention in object.